1. Field of the Invention
The present invention relates to an image forming apparatus such as a reproducing machine and, more particularly, to a color image forming apparatus of the type for forming an image on an image retainer by writing a plurality of images on the image retainer.
2. Description of the Prior Art
An image forming apparatus for color electrophotography or the like being developed is equipped at the circumferential edge portion of its image retainer with a plurality of developing devices, which are charged with toners of different colors such as red, blue and black colors so as to conduct multicolor developments. Latent images on an image retainer, which are sequentially formed in response to color signals coming from image reading units, are developed to compose a color toner image, which is transferred to a sheet of copy paper or transfer material to obtain a color copy.
If the latent images corresponding to the specified colors are formed on the image retainer by that developing treatment, a strong bias voltage is applied between the image retainer and the developing device charged with a developer of the above-specified color, i.e. a carrier and a toner of the specified color so that the toner of the specified color held by a developing sleeve in the developing device is transferred to the side of the image retainer.
On the other hand, since the image retainer is a drum-shaped member having a large capacity, as is well known in the art, it is desired to have its external diameter made as small as possible in not only the color type image forming apparatus but also a general image forming apparatus. Generally speaking, it is desired to make the length of the outer circumference of the image retainer slightly longer than the length of the maximum document to be used. If this desire is satisfied, it is possible to reduce the size of the image forming apparatus and to establish a quick copying cycle for continuous copies without specifically increasing the circumferential velocity.
Since, however, the image forming apparatus of the aforementioned color type has its developing devices of developers of the individual colors arranged in the direction of rotations of the image retainer, there follow respective time differences between the beginning and end of the development of each developing device with respect to the rotational period of the image retainer. Unless, therefore, the setting of the order of the developing devices for the development by applying bias voltages is proper, the timings at which the developing operations are required occur simultaneously for the two developing devices so that, while one developing device is developing the trailing end portion of a latent image, the other developing device develops the leading end portion of the latent image.
As shown in FIG. 6, more specifically, the construction is made such that individual developing devices I, II and III are arranged with respect to an image retainer D rotating in the direction of arrow. With this construction arrangement, the time periods for the image retainer D to reach individual developing sleeves s1, s2 and s3 from an exposure point P are designated at t1, t2 and t3, respectively, and the developing treatments are set in the order of the developing devices III.fwdarw.II.fwdarw.I. A time chart for the developing treatments according to the rotations of the image retainer is shown in FIG. 7, from which it is found that the timing for beginning the developing treatment is made gradually earlier to establish the timings where development processes with toners of two colors have to be conducted simultaneously, as indicated at a and b.
For producing a color copy on a sheet of recording paper, for example, according to another method: the colors of an image are separated; toner images of separated individual single colors are transferred to and fixed on sheets of recording paper; and these sheets of recording paper once copied are fed again so that subsequent toner image of single colors separated are superposed on a sheet of recording paper. However, the color copy thus obtained cannot avoid a drop of the copy quality and an offset of the image and still has a remarkably low copying speed per one sheet.
According to still another method, on the other hand, an intermediate member is provided in addition to an image retainer so that color toner images of individual colors are transferred to and fixed on a sheet of recording paper after they have been superposed on the intermediate member. This method cannot avoid an increase in the size of an image forming apparatus itself therefor and a complicated mechanism of the apparatus.
As image forming apparatus that are free from those defects, we have made proposals which are disclosed in detail in Japanese Patent Laid-Open No. 59-181362 (1984) and Japanese Patent Applications Nos. 60-192710 (1985) and 60-192711 (1985).
After having been transferred to a transfer material or a sheet of recording paper, a toner image has its residual toner peeled and removed therefrom as in an existing image forming apparatus by the action of a cleaning device. Since, however, the cleaning action is conducted by the pressure contact (including the abutment) of a blade member of another cleaning member with an image forming surface, as is well known in the art, it cannot be undergone during the formation or before the transfer of the toner image. In the meanwhile, therefore, the cleaning member is held in a position spaced from an image retainer so that the aforementioned pressure contact of the blade member is started immediately before the leading end of the toner image having been transferred reaches the cleaning device and is released immediately after the trailing end of the toner image has passed the same.
Thus, especially in the cleaning device used in the color iamge forming apparatus, the pressure contact and release of the blade member are conducted for the rotational period of the image retainer so that the speed of the rotation of the image retainer is caused to fluctuate more or less by the resultant frictional resistance. As a result, if the pressure contact and release of the blade member are conducted especially while a latent image is being formed on an image forming surface by an exposure, the circumferential speed of the image forming surface varies to cause disturbances due to the discontinuity of the image scanning operation, thus resulting in a deterioration of the image quality. This naturally can also apply to the monochromatic image formation which is accompanied more or less by a similar problem.